Such conditions may exist, for example, at the otput end of a fiber-optical signal path whose attenuation is to be determined by amplitude measurements. This is usually done by illuminating the input end of the optical fiber with a beam of light modulated by a sinusoidal signal of predetermined amplitude. The transmitted radiation is picked up at the opposite end by a photodetector working into a high-gain amplifier which suppresses the d-c component of the received signal and has a limited bandwidth (usually less than 1 KHz) centered on the nominal frequency of the sinusoidal modulation. Further narrow-band filtering serves to suppress the accompanying noise.
Analog filtering with tuned circuits of high figure of merit or Q-factor (about 10.sup.3) causes problems when the nominal frequency changes because of unpredictable influences such as temperature variations or component aging. The use of a crystal-controlled oscillator at the transmitting end is generally inconvenient, on account of the low modulating frequencies normally utilized, and mechanical frequency generators are cumbersome. The transmission of a phase reference to the measurement site would require a separate signal path.
Signals of high frequency stability can also be generated by a digital processor with recursive or transversal filtering. The frequency, in that case, depends solely on the recurrence rate of a train of clock pulses that can be derived from a crystal-stabilized oscillator through suitable step-down stages. Because of its complexity, however, such a circuit arrangement would not be very practical for the generation of the modulating signals at the transmitting end.